NEW YORK (CNN/Money) - There's a reason media executives are worried about the impact that General Motors' financial woes and Procter & Gamble's purchase of Gillette Co. could have on the advertising market.

GM, the world's largest automobile maker, and Procter & Gamble, the country's biggest seller of household products, together spent nearly $8 billion in advertising in 2004, according a survey released this week of the top U.S. advertisers.

GM ranked No. 1, with nearly $4 billion in ad buys last year, according to an annual Advertising Age survey of the country's top 100 marketers. Procter & Gamble finished second, having spent more than $3.9 billion on advertising.

Both GM and Procter & Gamble saw their advertising expenditures rise in 2004; GM by 6.3 percent and Procter & Gamble by 17 percent.

While good news for the media industry, the numbers do not tell the 2005 story for either company.

GM, which lost $1.1 billion in the first quarter, is slashing costs in the face of its worst financial crisis in more than decade. Procter & Gamble's deal to buy Gillette, the No. 41 top advertiser in the country, is renewing fears that the combined company will reduce its total ad spending.

If history offers any perspective, however, the bottom isn't going to fall out of GM and Procter & Gamble's advertising anytime soon. The two have dominated the Advertising Age rankings in 30 of the 50 years surveyed. In the first study published in 1955, the two dominated with just $255 million in advertising between them.

Last year GM and Procter & Gamble accounted for 8 percent of the $98.3 billion worth of advertising bought by the top 100 advertisers -- a dollar amount that covers all media, including what Advertising Age calls "unmeasured" advertising. In turn, the top 100 accounted for nearly 37.3 percent of total U.S. media buys in 2004.

Among the study's other key findings:

• While the top 100 advertisers are credited for pulling the U.S. media industry out of a recession in 2003, the growth in their overall expenditures in 2004 was slightly below the rest of the U.S. advertising industry.
• Advertising Age says the leading advertisers are diverting money into new media. Internet advertising by the top 100 grew about 41 percent in 2004, compared to 14.3 percent in 2003. However, the publication also reports that their spending on network television also grew last year, by 10 percent.
• The top five advertising categories, in order and by dollar volume are: financial services; health care; telecommunications; Internet services; and automotive.

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